The present invention relates to a rotary compressor for compressing helium gas and, more particularly, to a rotary compressor having a relief passage for preventing undesirable liquid compression in the discharge chamber of the compressor.
Helium refrigerators which make use of helium as the refrigerant have been used for ultra-cryogenic uses because such refrigerators can easily generate cryogenic temperatures on the order of -200.degree. C.
A compressor generally referred to as "rotary compressor", having an eccentric rotor rotatable in a closed space, is known as a compressor which is used in helium refrigerators. More specifically, this type of compressor has a cylinder which defines a closed space therein, a rotor adapted for eccentric rotation in the closed space while keeping a sliding contact with the inner peripheral surface of the cylinder, and a blade projecting into the closed space and contacting the rotor so as to separate the closed space into a suction chamber and a discharge chamber. In operation, the helium gas is drawn into the suction chamber the volume of which is progressively decreased so as to compress and then discharge the gas, as the eccentric rotation of the rotor advances.
On the other hand, the helium gas exhibits a drastic temperature rise when compressed, because it has a large adiabatic coefficient as compared with other types of refrigerant such as freon. For instance, when the compression is conducted at a compression ratio to the extent of 2 or 4, the helium gas drawn at about 30.degree. C. exhibits a high temperature of 200.degree. C. or higher when discharged. In consequence, the compressor which handles helium gas as the fluid to be compressed encounters problems or troubles such as reduction in the viscosity of lubricating oil due to high temperature, deterioration of the lubricating oil and so forth, with a result that the sliding parts of the compressor are not lubricated sufficiently. In the worst case, these sliding parts are damaged due to inferior lubrication.
To obviate these problems, it has been proposed to provide an oil injection mechanism in the compressor of the kind described. This oil injection mechanism is adapted to inject the lubricating oil into the drawn helium gas, after a cooling by a suitable means. This oil injection affords a remarkable reduction in the refrigerant temperature at the discharge side of the compressor. For instance, in the above-mentioned case where the helium gas sucked at about 30.degree. C. is compressed at a compression ratio of 2 or 4, the temperature of the helium gas at the discharge side is advantageously decreased from 200.degree. C. to 120.degree. to 130.degree. C.
The oil injection mechanism, which is effective in overcoming the problems concerning the temperature rise, produces another problem in that the rate of supply of the lubricating oil into the suction chamber of the compressor is increased impractically due to the injection of lubricating oil for the cooling purpose, and in that liquid oil compression inevitably takes place in the final stage of the discharge stroke. The liquid oil compression causes an abnormal pressure rise in the discharge chamber of the compressor, possibly resulting in a vibration and breakdown of movable parts such as the blade.
Some proposals have been made for preventing liquid oil compression in this type of compressor. For instance, Japanese Utility Model Application Laid-Open No. 66195/1983 discloses a compressor in which an axial recess is formed in the inner peripheral portion of the cylinder at a position between the discharge port and the blade so that the lubricating oil confined in the discharge chamber is received in the recess. This known arrangement is effective in the compressor which does not employ any oil injection system, e.g., the compressor which handles a different refrigerant such as freon. This arrangement, however, has only a limited capacity for holding the oil, and cannot prevent liquid oil compression when it is adopted in a compressor having the oil injection mechanism. Another measure for preventing the liquid oil compression disclosed in Japanese Patent Unexamined Publication No. 98687/1980 employs a circumferential groove formed in the inner peripheral surface of the cylinder and extending between the discharge port and the blade, whereby the oil confined in the discharge chamber is relieved through this groove. It is true that this arrangement can eliminate the risk of liquid oil compression in the discharge chamber. Unfortunately, however, this arrangement causes the high pressure oil introduced into the discharge port to act on a discharge valve, so as to abnormarily increase the velocity at which the valve member is brought into collision with the valve seat, causing a risk of breakdown of the discharge valve.
Thus, no practical measure has been established for effectively preventing the liquid oil compression in a rotary compressor having the oil injection mechanism.